RFR: 8317371: GenShen: Needs improved support for humongous allocation [v5]

[Kelvin Nilsen]

> Several improvements:
> 1. If there is a humongous allocation failure, go immediately to Full GC rather than trying 64 degenerated GC first
> 2. Reduce likelihood of humongous allocation errors by:
>     a. Allocating regular objects from top end of memory while allocating humongous from bottom end of memory
>     b. (Especially) in the case that regular regions that are promoted in place reside within the bottom end of memory, detect that old-gen memory has become fragmented and trigger old-gen GC, which will make a special effort to move these out-of-place old-gen regions to higher addresses.

Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 13 commits:

 - Merge master
 - Respond to reviewer feedback
 - Experiment with 0 for default humongous reserve
 - Cosmetic improvements
 - Fix assertion to deal with promote-in-place-only evacuations
 - Fix whitespace
 - Fix accounting of mixed evac regions added to defragment old-gen for logging
 - Cleanup code for upstream
 - Fix mismatched brace
 - Misc improvements to make humongous allocation support more performant
 - ... and 3 more: https://git.openjdk.org/shenandoah/compare/206e5b82...041abc3f

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Changes: https://git.openjdk.org/shenandoah/pull/336/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=336&range=04
  Stats: 237 lines in 11 files changed: 185 ins; 6 del; 46 mod
  Patch: https://git.openjdk.org/shenandoah/pull/336.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/336/head:pull/336

PR: https://git.openjdk.org/shenandoah/pull/336